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Potent Arctic blast

Published 2nd March, 13:06

High pressure will become centred over Iceland and Greenland during the coming days. That change allows a plunge of Arctic air to move southwards across the UK. It will become cold enough for snow, but how widespread will the risk become next week?

Most wintry spell since December

The chart below is generated using data from the UK Met Office Global model. It shows forecast surface pressure and air temperatures at about 1500m above sea level on Tuesday, 7th March. The blues indicate cold or very cold Arctic air sweeping down across the UK.

The general pattern is well supported by the main global computer models, but the details are differing. When looking more than a few days ahead that is to be expected.

Met Office, Global model 850hPa temperatures

In many ways the pattern is similar to the one which delivered the "Troll from Trondheim" cold period in December 2022.  However, the days are longer and the sun stronger at this time of the year. Therefore, temperatures will rise more quickly in the afternoon sunshine, if there is any to be had.

For example, in southern and central regions afternoon values could peak between 4C and 7C on some days. That will lead to lying snow thawing, but low dew points would slow down the process. Also, the nights are still quite long and there is the potential for some very low temperatures, especially if there is snow on the ground.  

The model data is suggesting the cold spell will last for most of next week in the northern half of the UK. Things are more uncertain in the south and there are indications of milder air trying to return from the southwest. However, that could also lead to the risk of snow becoming more widespread.

How widespread is the risk of snow?

Winds are expected to be blowing from a north or northeasterly direction, so most of the showers are likely to be in the north and east of Britain. At this time of the year it will be difficult for them to produce anything other than transient snow cover in the southern half of the UK, but it's a very different story over high ground in the north. 

Despite that, there are other routes for accumulating snow to become more widespread, at least for a time. For example, disturbances in the northeasterly flow may develop and move down across the UK.

Also, there is the possibility of low pressure systems pushing up from the southwest. They would bring mild and moist air which would result in more persistent periods of precipitation. As the mild air bumps into the cold air over the UK the precipitation is likely to fall as snow down to low levels close to the border zone. If the border zone becomes slow moving heavy falls of snow close to could be the result.

The ECM chart below for Friday, March 10th illustrates this type of scenario. (Important note: I am showing it only for illustrative purposes. It is much too far away to be focusing on the details).

A small area of low pressure is tracking eastwards across central Britain. Would it bring rain or snow and if so then where?  

The chart below is also generated using data from the ECM model. It shows forecast temperatures at about 1500m above sea level.

The key point is they are above 0C in southern and central counties which means precipitation would be falling as rain. The boundary between air masses lies across northern England, Northern Ireland and southern Scotland, so these are the areas where outbreaks of snow would be expected at this time. 

Nonetheless, if the low pressure tracked 150 miles further north or south the area of snow would move commensurately. On the global scale 150 miles is a tiny adjustment for computer models and that is one of the reasons why it is impossible to forecast the details at this range. Another is that at the present time the area of low pressure only exists in the simulated world of the computer model. It may not even develop in the real world!  

Summary

A cold and wintry period of weather starting this weekend and lasting for most of next week is consistently being forecast by computer models. Snow showers are most likely in the north and east of the UK, but accumulations in the south would probably be transient.

However, there is a possibility of longer periods of rain, sleet and snow. These could be the result of disturbances in the northeasterly flow moving down across the UK or areas of low pressure pushing up from the southwest.

At this stage it is impossible to be confident about the forecast details so it is very much a case of staying up to date with the short range forecasts as things begin to firm up. The ingredients for disruptive snowfall are falling into place, but whether they will correctly mix for it to be the outcome in large parts of the UK is a different question.  

Latest charts

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